Gravitational Forces

Science — First Preparatory

Mass and weight

Feature	Mass	Weight
Definition	The amount of matter in an object	The pull of the Earth (or another planet) on the object
Unit	Kilogram kg	Newton N
Does it change with place?	No	Yes, when gravity changes
Instrument	Two-pan balance	Spring balance

A spring balance reads the weight directly in newtons, whereas a two-pan balance compares the mass with known masses, so its reading of the mass stays the same anywhere.
A student can remember that mass is the same everywhere, while weight changes.
The relation between weight and mass is Weight W = mass m × gravitational acceleration g. The unit of g is N/kg or m/s². On the Earth’s surface g is about 9.8 N/kg, and on the Moon’s surface g is about 1.6 N/kg.

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